Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate.
Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask that is placed between the light source and the plate. With the advance of laser and computer technologies, laser sources have been increasingly used to directly expose a lithographic printing plate that is sensitized to a corresponding laser. This allows the elimination of the photomask film, reducing material, equipment and labor cost. Among the lasers used are infrared lasers (about 830 nm or 1064 nm), FD-YAG laser (about 532 nm), argon ion laser (about 480 nm), and violet laser (about 405 mm). Laser diodes (such as violet laser diode of about 405 nm, and infrared laser diode of about 830 nm) are especially useful because of their low imager cost.
Among the laser sensitive plates proposed (including silver halide plates, positive plates, and negative plates), photopolymerizable composition based (also called photopolymer) laser sensitive plate is the most desirable because of the high durability of conventional photopolymer plate and its negative-working characteristics (no coated edge on the developed plate when imaged on external drum). However, because the energy output of a laser diode is quite limited and the exposure time of laser for each area is extremely short, it is very difficult to design a laser sensitive photopolymer plate with sufficient photospeed and press durability suitable for commercial applications.
U.S. Pat. No. 6,689,537 describes a violet photopolymer lithographic printing plate where a grained and anodized aluminum with higher reflection optical density (from 0.3 to 0.5, by using no or less desmut treatment) is proposed for achieving higher photospeed and improved printing durability. The aluminum substrate is electrochemically grained and anodized, with no or less desmut treatment, and then directly coated with the photosensitive composition. While such a substrate gives higher photospeed and improved press durability, it has the tendency of background toning (inked non-imaging areas) on press, compared to typically clean background for substrate with typical optical density (such as 0.20 to 0.28, with typical desmut treatment). Hydrophilic treatment after the anodization is not used apparently because the hydrophilic treatment generally reduces the photospeed and press durability.
The inventor has found, surprisingly, laser sensitive photopolymer lithographic plate based on hydrophilically treated electrochemically grained aluminum substrate with an optical density of at least 0.30 can give excellent photospeed and press durability while having clean background. The combination of higher optical density and hydrophilic treatment is unexpected for laser sensitive photopolymer plate, because higher optical density is expected to increase photospeed and press durability while hydrophilic treatment is expected to reduce photospeed and press durability; in addition, such darker substrate absorbs more laser energy and is considered less desirable in terms of utilizing the reflected laser to enhance photospeed, and is less desirable for inspection purpose because it gives lower contrast between the imaging areas and background areas of a processed plate.